Sub-acute rumen acidosis (SARA) can be induced by feeding high amounts of concentrates to dairy cows. During SARA, the pH value in the rumen decreases and can reach values below 5.6. These conditions lead to the release of high amounts of endotoxins (Gozho et al., 2006, 2007; Li et al., 2012). If endotoxins reach the blood flow, through an impaired rumen barrier, they can induce the release of pro-inflammatory mediators and the production of acute phase proteins (Gozho et al., 2006; Li et al., 2012). The aim of the presented study was to evaluate, if a clay-based product reduces the endotoxin concentration in a semi-continuous in vitro rumen model. For this purpose, rumen fluid was sampled at a slaughterhouse. For each trial, rumen fluid from 3 different dairy cows was pooled, and immediately transported to the lab. Rumen fluid was incubated at 39°C under anaerobic conditions for 360 h (4 reactors per treatment, 3 independent trials). Each reactor contained 1.25 L of the inoculation mixture (50% rumen fluid, 30% distilled water, 20% synthetic saliva). Turnover of rumen fluid was maintained by constant inflow of synthetic saliva. Feed was provided every day with a nylon bag (43% chopped hay, 57% concentrate). The clay-based product (0.3%) was added daily to the reactors. To evaluate endotoxin concentration in the rumen fluid, samples were taken at 48, 168, 240, and 360 h. The limulus amoebocyte lysate (LAL) assay was used for analysis. GraphPad Prism software was used for statistical evaluation of results. If data were normally distributed, Student’s t-test was used. If data were not normally distributed, the Mann Whitney test was used. The clay based product was able to reduce the endotoxin concentration by 56% at 48 h (P = 0.0035), 76% at 168 h (P = 0.0002), 77% at 240 h (P = 0.0000), and 77% at 360 h (P = 0.0002). The results of the presented study revealed that the clay-based product was able to reduce the endotoxin concentration in in a semi-continuous in vitro rumen model. However, in vivo trials are necessary to confirm these results.